A typical example of the conventional tubular membrane module has a plurality of porous support tubes each containing a semipermeable membrane which are arranged parallel in a plurality of stages, and which are received in a cylindrical casing and are provided at both ends thereof with heads containing return bends so as to connect the porous support tubes into zigzags, and the heads and the porous support tubes are fixed together by fastening them by means of a stay bolt passing through the heads.
However, in this type of the conventional tubular membrane module, in order to receive a multiplicity of the porous support tubes each containing a semipermeable membrane into the cylindrical casing as many as possible effectively, such following problems were caused depending on the number of the porous support tubes or the number of the stay bolts received in the casing of the same capacity.
Namely, a typical example of the conventional tubular membrane module has, as shown in FIG. 1 or 2, a multiplicity of porous support tubes 1 each containing a semipermeable membrane 2 (all of the tubes contain semipermeable membranes but FIG. 1 shows only one porous support tube containing the semipermeable membrane in view of simplification of FIG. 1) which are arranged parallel in a plurality of stages into a casing 3, and which are fixed at both ends thereof with heads containing return bends so as to connect the porous support tubes, and the heads and the porous support tubes are tightly fastened together by means of one or two stay bolt(s) 5.
In this constitution, the porous support tubes 1 are arranged symmetrically and the number of the stay bolt is selected to be one (FIG. 1) or two (FIG. 2).
In case of using a single stay bolt disposed at the center of the casing as shown in FIG. 1, the same number of the porous support tubes (two in this case) are disposed at both sides of the stay bolt in any radial directions, so that as a whole an even number of the porous support tubes (eighteen in this case) are disposed in the casing and further an inlet A and an outlet B are disposed at the same head side, thereby simplifying the arrangement and maintenance of the support tubes. However, since the stay bolt 5 passes through both the heads, the porous support tubes 1-a and 1-b at both sides of the stay bolt 5 can not be directly connected through a return head contained in the head. Thus, in order to connect the porous support tubes 1-a and 1-b, such a fluid path of a tube (1-a).fwdarw.a tube (1-c).fwdarw.a tube (1-d).fwdarw.a tube (1-e) a tube (1-b) is formed so as to go around the stay bolt as shown in FIG. 1. In this fluid path, in case of removing the solution remained in the tubes from the inlet A after completion of the treatment of the solution, a solution stagnant portion U can be disadvantageously formed between the tubes 1-c and 1-e because there is a solution rising path from the tube 1-c to the tube 1-a.
As a direct influence due to the presence of the solution stagnant portion U, the solution therein is likely to be rotten. Further, as an indirect influence due to the presence thereof, when the operation of the separation apparatus is stopped, the solution in the solution stagnant portion is transmitted out of the tube through the tubular membrane to reduce the volume of the solution in the solution stagnant portion to reduce the inner pressure within the tubular membrane, thereby exfoliating the membrane from the inner surface of the porous support tube or breaking the membrane to damage the membrane. Thus, upon the next operation of the separation apparatus, the solution is likely to be leaked from the damaged portion of the membrane or the transmission efficiency of the membrane is likely to be deteriorated thereby deteriorating the functions of the separation apparatus. (In the drawings, relating to the connection between the inlet A and outlet B through the return bends, a solid line designates the connection by the return bend in the front side of of the drawings and a dotted line designates the connection by the return bend in the rear side thereof.)
On the other hand, in case of using two stay bolts 5 as shown in FIG. 2, the two stay bolts are disposed at both ends of a series of porous support tubes alligned at the center of the cross section of the casing. In this case since a space for passing the two stay bolts is required, the number of the porous support tubes between the stay bolts 5 is required to be less by one when compared with FIG. 1 (three in FIG. 2, while four in FIG. 1) supposing that the size of the casing 3 and the porous support tube 1 in FIG. 2 is same as those in FIG. 1). Thus, the total number of the porous support tubes is an odd number (seventeen in this case), so that there is not such a solution stagnant portion in the solution path as shown in FIG. 1. However, there are such problems that the number of the porous support tubes, i.e. the area of the tubular membranes is smaller than that of FIG. 1 and that the inlet A and outlet B are disposed at different heads since the total number of the porous support tubes is an odd number.